1. Field of the Invention
The present invention relates to a process and device for producing screw connections and can be used preferably in assembly processes in industrial production, workplace operation and in monitoring in plant engineering where there are high safety requirements.
2. The Prior Art
It is known from the prior art that in industrial manufacturing electronically controlled screw systems are being used more and more frequently. These comprise a drive unit and a tool spindle with a screw tool which screws in a screw with a predetermined screwing torque in order to press together the components to be joined together with a preset force. This force is designated as prestress force and originates from the change in length of the screw shank, designated hereinafter as elongation. In many applications it is necessary to adjust the prestress force to a predetermined value as precisely as possible. For this, it is required to determine the prestress force. It is known from the prior art to determine the prestress force indirectly by means of measuring the screwing torque during screwing. It is mainly torque or angle of rotation measuring systems integrated into the screw system which are utilised for making a screw connection. When a pre-adjusted torque is attained in screwing in the screw, a signal emitted by the torque measuring system is used for switching off the drive unit, or the screw is turned further about a preset angle of rotation, determined by the angle of rotation measuring system.
The following problems arise with indirect determination of the prestress force by torque measurement: as a screw is being tightened, a small portion only of the applied torque for the elongation of the screw, that is, for the production of prestress force, has an effect. The majority of the torque is absorbed by the friction on the screw thread and on the screw head. If the friction conditions are changed, these changes have a strong effect on the prestress force. Therefore, the abovedescribed devices cannot guarantee that narrow tolerances of prestress force are adhered to.
In order to overcome these difficulties, the prestress force has to be measured directly. Different processes and devices have been put forward for this purpose. A measuring ring in the form of a plain washer is known to be arranged underneath the screw head. This measuring ring is a sensor which gives off an electrical signal with the action of force. If the screw is tightened, the screw head presses on the measuring ring, and the prestress force can be measured directly. This process is very cost-intensive, since the measuring ring remains under the screw head following tightening of the screw connection. This process is accordingly restricted to special applications such as, for example, aeronautics or nuclear energy technology. Ongoing long-term monitoring of the contact force with measuring rings is only partially possible, as the latter comprise an uncontrollable null drift.
DE 4017726 describes a fastening screw having a shank provided at least partially with a thread and an actuating end on which a head, a bolt stop or the like, whereby a first end surface is formed on the actuating end of the fastening screw and a second end surface is formed on the free end, and measuring surfaces for ultrasound measuring are provided on both end surfaces, which extend over only a portion of the end surfaces and are arranged offset axially to the end surfaces in terms of an elevation and/or a depression. In this device the change in length of the screw is measured by ultrasound. By means of methods known to the specialist the prestress force is determined from the characteristic values of the materials and the geometric dimensions of the screw.
DE 19507391 concerns a screwing device for ultrasound-controlled tightening of screw connections.
Still another realization of ultrasonic measurement is shown in DE 4025430 concerning a torque controlled screwing head having an electroacoustic sensor to measure distortion.
Determining the change in length of the screw by using ultrasound does, however, have drawbacks. To detect the change in length with precision, the ultrasound has to be introduced into the bolt or screw in a defined manner. The technical problems to be solved in this process are considerable.
In particular, the bolt or screw to be used has to be provided with a special design of a ultrasound-initialising surface and a reflection surface. These surfaces need to be manufactured with high precision within close tolerances. Such requirements cannot be met by mass-production like that of standard screws. Thus, manufacturing costs for special screws to be used in processes comprising the measurement of prestress forces by ultrasound are considerable.
The use of laser beams for monitoring forces effective in specially designed bolts is disclosed in SU 1687423. This known monitoring does not fit to the production of screw connections under control of prestress forces.
DE 26045510 describes a process for determining the binding power by means of a bore reaching from the screw head axially into the screw shank and by means of a metering gauge inserted into the bore measuring the expansion of the screw shank. The drawback to this process is that contacting mechanical measuring for serial application is error-prone and inaccurate.
A similar process to this is described in EP 0182185, comprising a lock screw with a pressed-in pin which moves as the screw is being tightened and whose movement is measured mechanically.